Elastography generally refers to methods of imaging mechanical properties of tissue such as elasticity, viscosity, relaxation time, shear modulus, porosity, etc. Elastography is generally carried out in four steps:                1. exciting the tissue by causing some form of deformation or movement in the tissue;        2. observing and recording with a medical imaging device a series of images depicting the motion of different locations in the tissue over an interval of time;        3. estimating the displacements of the tissue at the different locations and time instances from the series of images; and        4. estimating mechanical properties of the tissue such as elasticity and viscosity from the estimated displacements.        
Numerous elastography systems have been proposed in the art by combining different types of excitation, with different imaging modalities. Known imaging modalities include ultrasound and magnetic resonance imaging (MRI), as well as optical coherence tomography (OCT) and x-ray computed tomography (CT). Different methods have also been proposed for estimating the displacements, and estimating elasticity and viscosity from the estimated displacements.
The majority of the magnetic resonance elastography (MRE) methods in the art use steady-state excitation, although transient excitation has also been studied for use in MRE. The majority of the ultrasound elastography methods in prior art use transient excitation, although steady-state excitation has also been studied.
Compared to MRI, OCT or CT, ultrasound imaging has certain advantages, such as lower cost, lighter weight and easier operation. However, existing real-time ultrasound elastography systems that provide imaging of tissue properties use techniques that are computationally intensive and which require sophisticated and expensive computing hardware, or which acquire images only in a 2D imaging plane. Such 2D measurements introduce errors or diminish the ability to measure the absolute value of elasticity, and instead measure just relative variations throughout an image. Therefore, existing ultrasound elastography systems using probes which only acquire data in a 2D imaging plane tend to produce inaccurate measurements.